Antimicrobial Activity, and Defense Gene Activation by Natural Extracts: Toward Sustainable Plant Disease Management

The growing demand for sustainable agricultural practices has accelerated research on eco-friendly plant disease management strategies, focusing on natural substances like plant extracts with bioactive compounds for antimicrobial activity or plant defense-inducing properties. This study evaluates natural extracts as sustainable alternatives for managing plant diseases. A variety of substances, including essential oils, extracts from ornamental and edible plants, by-products from beer and coffee processing, Aloe vera extracts, and selected commercial formulations, were assessed for their antimicrobial efficacy through in vitro and in vivo assays. Several extracts demonstrated notable bioactivity: Agapanthus africanus showed significant antifungal effects against the fungus Botrytis cinerea, while essential oils from thyme, tea tree, and lavender were effective against both B. cinerea and the bacterium Pseudomonas syringae pv. tomato. Greenhouse trials on tomato plants revealed the protective effects of Aloe vera polysaccharides and ornamental plant extracts, particularly Eremophila nivea and Limoniastrum monopetalum, against Potato Virus Y. Additionally, microencapsulated thyme essential oil and alginate-based formulations showed enhanced in vivo performance, particularly against the bacterial and viral pathogens. Gene expression analyses at 12- and 72-hours post-treatment indicated strong upregulation of the defense-related gene PR1, with extracts from coffee residues, A. vera gel, and the synthetic resistance inducer BION®, this latter used as reference, eliciting the most pronounced response, while PR4 exhibited a more moderate induction. These findings highlight the promising role of natural plant-derived and agro-industrial substances in providing both direct antimicrobial action and stimulation of plant immune responses, offering valuable insight into sustainable disease management strategies.